The present invention relates to the field of semiconductor processing equipment and more specifically to a method and apparatus for reducing residues inside a semiconductor processing chamber.
During chemical vapor deposition (CVD) processing, deposition gas molecules are deposited on the surface of a substrate. Some of these molecules also come in contact with areas of the chamber, such as the aluminum walls, resulting in the unwanted deposition of material and residues. When the build-up of surface deposits on the inside of the processing chamber becomes thick, flakes or particles of the deposited material can break off from the surface of the chamber onto the substrate being processed, potentially causing a defect. Surface deposits also adversely effect other processing conditions such as deposition uniformity, deposition rate, film strength and the like.
To avoid this problem, the inside surfaces of processing chambers are periodically cleaned to remove the material deposited by the deposition gases. Various techniques have been used or proposed for cleaning residues from interior surfaces of a processing chamber. One conventional cleaning technique involves physically cleaning the interior surfaces of the processing chamber by way of rinse water and clean wipes. A limitation with this technique is that the processing apparatus often operates no more than 1000 to 2000 runs before substantial residues build up on its interior surfaces. To remove these deposits with a rinse water and clean wipe requires that the processing apparatus be taken down to clean its interior surfaces. A further limitation is that this conventional cleaning process often takes several hours or more to perform. Taking a machine down for hours at a time clearly impacts wafer throughput in the fabrication plant.
Another technique suggests the use of etching gases, such as fluorine, for removing deposited material and residues from the chamber walls in a plasma process. Detailed descriptions may be found, for instance, in U.S. Pat. No. 4,960,488, U.S. Pat. No. 5,124,958 and U.S. Pat. No. 5,158,644. The plasma enhanced etching gases can be used periodically after a group of N wafers has been processed or after every wafer. The etching gases react with the deposited material so as to remove it from the surfaces of the processing chamber. Such techniques, however, often cannot remove all the aluminum and organic residues from the chamber walls without other detrimental effects. In fact, the etchant gases can increase the amount of particles into the processing chamber from precipitates derived from the etching gases and the like. Examples of precipitates include aluminum oxide, aluminum fluoride, aluminum oxy-fluoride, and others. The precipitates increase the particle count in the processing chamber, which is clearly an undesirable result. Moreover, the use of such etchant gases in a clean step requires additional processing time thereby decreasing wafer throughput.
The physical cleaning techniques described above are often used in conjunction with etching gases to periodically clean the residues and precipitates which remain after a chamber clean. However, as described above, the physical clean process results in further down time of the processing machine and lower wafer throughput.
Thus, it is desirable to keep processing chambers clean and substantially free from particles to prevent contaminating semiconductor integrated circuits being processed and to minimize down or cleaning time so as to increase wafer throughput as much as possible. From the above it is seen that an apparatus for processing semiconductor wafers that requires less periodic cleaning is desirable.
One approach to eliminate residues left over from a chamber clean has been utilized in a plasma enhanced chemical vapor deposition (PECVD) silicon nitride processing chamber as shown in cross section in FIG. 1. The chamber is cylindrical in shape. The chamber includes a heated pedestal 10 for supporting a substrate or wafer 12 for processing and a shower head 14 for dispersing process gases above the wafer under plasma conditions. The plasma is formed between shower head 14 and substrate 12 by using an RF source 16 to apply a potential to the shower head, with the pedestal 10 being grounded. In addition, chamber walls 18 are also grounded. The chamber walls are insulated from the voltage applied to the shower head by RF insulators 20. Since this is a cross section, RF insulator 20 is a ring around the circular shower head 14.
The process gases are applied through shower head 14 to wafer 12. An exhaust channel 22 is formed in the chamber wall, as an annular ring. At one point, there is a port 24 connected to a vacuum pump to draw out the exhaust gases.
In one example, a plasma enhanced CVD film of silicon nitride is deposited on wafer 12 using a recipe requiring silane, N.sub.2 O and ammonia or a similar recipe. The deposited nitride film forms on the surface of wafer 12. Unfortunately, films can also deposit on the chamber itself, forming an undesirable residue which could contaminate subsequent process steps. The primary area of deposits are on the chamber walls adjacent the exhaust channel 22. Because gas is being forced through the gas distribution manifold, less of the reactant will form on the gas distribution manifold itself, since it is directed away by the incoming gas. In addition, the gas distribution manifold is hotter than other parts of the chamber, since it is in contact with generated plasma, and the heat reduces residue build-up. With respect to the pedestal, most of this is covered by the wafer substrate, and thus there is less formation of residue there. With respect to the lower parts of the chamber walls, in one implementation, a purge gas is forced up along the sides of pedestal 10 between it and the chamber walls 18 along a path 26. The purpose of the purge gas is to avoid deposits at undesirable locations and prevent deposits on the wafer edges. Accordingly, this is one method of protecting the lower parts of the chamber wall from undesirable residue buildup. Since the vacuum pump draws the exhaust gases, this is a primary area where residue will build up on the chamber walls. RF insulator 20 is not connected to ground, and thus less of the plasma forms between it and the shower head, reducing the amount of residue buildup.
An in-situ plasma clean is typically used to clean these deposits. Typically, the in-situ clean contains a fluorine species such as CF.sub.4 and an oxide such as O.sub.2 which are ignited under plasma conditions. The clean typically removes material deposited during the nitride deposition step, but leaves its own residue as described above. The etched away material is pumped by a pumping system, not shown, through exhaust port 24.
The residue left over from the chamber clean is made up of particles from some or all of a combination of oxygen, nitrogen, silicon, fluorine and similar elements. Heaters 28 heat walls 18 of the processing chamber during the clean step to above 100 degrees C. Such an elevated temperature deters deposition of the process and exhaust gases that may otherwise form on the chamber walls. The inclusion of heaters 28 adds to the complexity and cost of the processing chamber and requires energy during operation which further increases processing costs. Without the heater, the aluminum chamber walls are typically around 70.degree.-80.degree. C. due to water cooling during processing.
U.S. Pat. No. 5,366,585, assigned to the same assignee as the present application, discloses the use of a free-standing ceramic liner around portions of the chamber wall exposed to the processing plasma. The ceramic was found to act as an insulator, reducing the build-up of residue on the electrically conductive aluminum chamber walls. It was disclosed that the ceramic barrier material need not shield the entire inner surface of the process reactor chamber, but it should protect reactor chamber surfaces surrounding the area in which plasma will be generated. The liner was preferably 130 micrometers to 6.4 millimeters thick, and made of a non-porous, homogeneous material, such as ceramic alumina.